X-ray lithography utilizes a variety of sources including X-rays emitted from a small area (point-sources) and synchrotron generated X-rays to generate an image. Unfortunately, X-ray lithographic systems have been limited by the inability to adequately manipulate the X-ray beam.
X-ray optics incur several difficulties not encountered in the visible or infra-red (IR range). Refraction in passing through media of a different refractive index cannot be used because of the strong absorption of photons with sufficient energy to excited or ionize electronic levels inside the media. Diffraction and interference phenomena can be used to deflect X-rays using Bragg scattering in single crystals, in multi-layer mirrors or by using zone and phase plates. Although these approaches are useful in many applications, they are very energy (wave length) selective and cannot be used to control X-ray beams having a broad energy spectrum. The use of reflection has also been limited because surfaces of all known materials have very low reflection coefficients for X-radiation at large angles of incidence.
Grazing-incidence optics have been developed based on the phenomenon of total external reflection of X-rays. This is widely used in synchrotron radiation facilities where flat mirrors are used for deflection and curved mirrors are used for focusing parallel beams. These mirrors typically use a single reflection. Such devices have an extremely small angular aperture due to the small value of the total-external-reflection angle (milliradians at KeV energies). Point-source X-ray lithography using existing equipment is limited by the following:
Intensity. The sources currently in development lack the intensity to achieve an exposure time which approaches production requirements. Modifications attempted to increase intensity are not only expensive, but by pushing the sources harder potentially decrease reliability, reduce source life and increase debris generation at the source which can damage the mask. PA1 Radial magnification. Because the beam from the source to the mask is divergent there is increasing distortion as the edge of the field is reached. Distortion may be reduced by adjusting the feature size and shape in the mask. Unfortunately, as gap tolerance becomes more critical, mask and wafer flatness requirements increase, alignment becomes increasingly difficult, field size is limited, and the same masks cannot be used on a synchrotron. PA1 Penumbral blur. The sources have a size large enough that illumination of the mask by different points of the x-ray generating area produce a blurring of the features projected on to the wafer. This lack of definition in the edge of the images projected limits the achievable minimum feature size. PA1 Source position instability. To the extent that X-ray spots are not be in the exact same position each pulse, feature patterns projected on the resist have decreased definition.
Synchrotron-source X-ray lithography is not intensity limited and has a beam which does not show significant divergence of any significance in the vertical direction. The beam, however, is very flat, normally 0.5-2.0 mm thick, is horizontal, and has a divergence in the horizontal plane which can be 6 degrees or larger. Because the beam is flat and the area to be exposed can be multiples of a cm square, either the wafer and mask must be moved to get a scan or a mirror in the beam line must be rotated to cause the beam to scan across the desired area. Horizontal beams require that the masks and wafer be vertical rather than horizontal as is more commonly used with optical steppers. The horizontal beam divergence causes the majority of the beam to be wasted with only a small portion of the beam reaching the mask and wafer at the end of the long beam lines.
The subject invention provides a solution to the long felt need in the art for an improved system of X-ray lithography. The subject invention provide the benefits of improved X-ray control, precision and accuracy.